I'm not sure if this had been an issue already but here it is.
After the fall of fort Henry US navy undertook what became known as timberclad raid. Foote sent Lieutenant Phelps with the three timberclads, the Tyler, Conestoga, and Lexington, on a mission up river to destroy installations and supplies of military value. The raid reached as far as Muscle Shoals, just past Florence, Alabama, the limit of navigability.
In this game ships are able to navigate up to Knoxville, TN. If forts Henry and Donelson are captured rebels are forced to abandon Nashville and retreat all the way back across the Tennessee River or risk to be cut off by US navy.